The present invention is directed to microporous hollow fibers having improved characteristics of strength and rigidity and to the process of forming the same. More particularly, the present invention relates to microporous, highly filled polyolefinic hollow fibers having structural strength and rigidity capable of inhibiting the collapse of the fiber and exhibiting porosity of extremely small pores and of narrow pore size distribution.
Hollow porous fibers are well known for their usefulness as filter medium. The advantages of permeable hollow fibers or permeable films are also known as they possess a larger surface area per unit volume than a flat film of similar porous configuration.
Filtration devices have been used to separate bacteria, proteins, viruses, colloidal substances, and the like. Ultrafiltration devices normally used for such separations have used membranes made from various materials such as gel cellophane films, cellulose acetate membranes, etc. However, these conventional membrane filters have various drawbacks such as low water permeability and low surface area per unit volume due to their normal flat configurations. Further, certain of these filter membranes are not stable to the environment to which they are required for use.
Hollow fibers which are porous have been produced from various polymeric materials such as polyacrylonitrile, as described in U.S. Pat. No. 3,877,955 and polyolefins, such as described in U.S. Pat. Nos. 4,020,230; and 4,401,567 and 4,405,688. Each of these materials are formed by a method which requires formation of the hollow fiber and subsequent inducing porosity in the fibrous materials by stretching, and annealing of the material after formation. It has been found that the pores and hollow fiber structure products tend to collapse under filter conditions normally encountered.
It is highly desired to have a polyolefin based hollow microporous fibrous material for use as a filtration medium in that polyolefins are a substantially inert compound with respect to a wide variety of materials requiring separation by such technique. It is thus highly desired to have a polyolefinic hollow fiber product which is capable of maintaining its porosity and stability during filtration.